Preheat oven to 375F.,Butter an 8-inch square baking pan, then line bottom with wax paper and butter paper.,Melt chocolate and butter in a metal bowl set over a saucepan of barely simmering water, stirring, until smooth.,Remove bowl from pan and whisk 3/4 cup sugar into chocolate mixture.,Whisk in whole eggs and salt, then sift cocoa over and whisk until just combined.,Pour batter into baking pan, spreading evenly, and bake in middle of oven until a tester comes out with a few crumbs adhering, 20 to 25 minutes.,Cool cake in pan on a rack 5 minutes, then invert onto rack and cool completely.,Cut cake into 6 equal pieces, then arrange 1 piece in each gratin dish, trimming to fit.,Top each piece of cake with a large scoop of ice cream (about 1/2 cup), then freeze, covered, just until ice cream is hard, about 25 minutes (do not let ice cream become rock hard unless making ahead see cooks' note, below).,Preheat oven to 450F.,Beat egg whites and a pinch of salt with an electric mixer until they just hold soft peaks.,Add remaining 1 1/2 cups sugar a little at a time, beating at high speed, and continue beating until whites just hold stiff, glossy peaks, about 5 minutes in a standing mixer or about 12 minutes with a handheld.,Remove gratin dishes from freezer and mound meringue over ice cream and cake, spreading to edge of gratin dish.,Bake on a baking sheet in middle of oven until golden brown, about 6 minutes.,______